Opacifiers for paints and coatings

ABSTRACT

Enhanced hiding power for opacifiers in paints, inks, and other coatings is provided by employing the opacifiers, such as TiO 2 , ZnO, talc, CaCo 3 , and the like, adhered to or embedded in the surface of thermoplastic microspheres. The composite opacifier-microsphere material is readily formed with a preselected specific gravity, so that there is little tendency for the material to separate. In addition, the composite behaves in the coating as a opacifier-air interface, having a very high difference in refractive indices, and exceptional hiding power. The high volume to weight ratio of the composite affords very attractive economic advantages, effectively reducing the weight proportion of opacifiers required in the formulations, and other related advantages.

RELATED CASES

Application is a continuation in part of the prior application ofMelber, Wolinski, and Oswald, S.N. 028,119, filed March 19, 1987, forCOMPOSITION AND PROCESS FOR DRYING AND EXPANDING MICROSPHERES, now U.S.Pat. No. 4,772,943, issued Feb. 2, 1988.

This application is also a continuation in part of the copendingapplications of Melber, Wolinski, and Oswald, S.N. 103,203, filed Oct.1, 1987, for THERMOPLASTIC MICROSPHERES, and S.N. 103,204, filed Oct. 1,1987, for SYNTACTIC POLYMER FOAM COMPOSITIONS CONTAINING MICROSPHEREFILLERS, each of which is a division of the aforementioned S.N. 028,119.

Benefit of each of the foregoing applications is hereby claimed underthe provisions of 35 U.S.C. 120.

BACKGROUND OF THE INVENTION TECHNICAL FIELD

The technical field of the present invention is coatings and paints andto new and improved opacifiers affording enhanced hiding power and ofopacifier materials having a specific gravity controllable to valuessignificantly lower than typical in the prior art and oftensubstantially the same as the coating vehicle and thus having littletendency to settle or float or otherwise separate from the coatingformulation.

SUMMARY OF THE INVENTION

The present invention, in specific terms, relates to composite materialsfor use as opacifiers in a wide variety of coatings, and diverse typesof coatings, paints, and the like.

More particularly, the present invention relates to a composite of dry,expanded thermoplastic microspheres having adhered to or embedded in thesurfaces thereof inorganic opacifier materials, such as titaniumdioxide, zinc oxide, calcium carbonate, talc, and the like.

Still more particularly, the invention relates to such composites wherethe proportions of the microsphere component and the opacifiers, and thedegree of expansion of the microspheres can be balanced to afford aspecific gravity of the composite material as necessary to preventfloating or settling in the continuous phase of the paint vehicle, i.e.,generally within the broad range of about 0.1 to 2.8 gm/cc, andpreferably about 0.15 to 1.5 gm/cc.

PRIOR ART

A wide diversity of opacifiers are known to paint and coatingtechnology, and the present invention can be employed, with suitableadjustments for the specific characteristics of the materials selected,with any of them.

As those of ordinary skill in the art will well understand, the presentinvention will be described in relation to the most commonly employed ofsuch materials. While this is not intended to exclude the employment ofstill other opacifier materials, it is believed that the full nature ofthe present invention and the parameters which should guide the art inits use will be most conveniently and fully understood in relation tosuch materials. These include, as mentioned above, such materials astitanium dioxide, in both anatase and rutile forms, zinc oxide, calciumcarbonate, talc, and where appropriate to the discussion, other relatedmaterials.

The opacifiers of the prior art have specific gravities which aregenerally quite high, ranging from about 1.75 up to as much as about4.5. As those of ordinary skill in the art are well aware, thesematerials have a decided tendency to separate from the medium andsettle, often as a hard settlement.

It is also common, in order to achieve the opacities desired, to employthe opacifiers in quite substantial proportions, most often in excess ofthe "critical pigment volume" required in order to achieve a degree ofporosity which produces a pigment or opacifier air interface whichenhances the opacity of the coating when dry. Such porosity, however,can lead to infiltration of environmental liquids, i.e. water from rainor the like, which operates to displace air from the pores, wet thepigment, thus reducing the effectiveness of the opacifier and greatlyreducing the durability of the coating.

The role of opacifiers in coating and paint technology are generallywell understood by practitioners in the art. The opacifiers of thepresent invention are well behaved specimens within the spectrum offamiliar, and well known materials, and will give those of ordinaryskill in the art little difficulty once the central properties andparameters which distinguish the materials from the conventions of theknown materials are clearly defined and understood.

As the art well understands, it is ordinarily the difference inrefractive indices between the coating binder and the opacifier materialwhich dictates the hiding power of a particular coating. That is, thegreater the difference in refractive indices at each occurance ofinterface between the coating binder and the opacifier material, thegreater the hiding power of the coating. Coating binders have refractiveindices which typically are in the vicinity of about 1.5 or 1.6.Opacifiers are most often materials having refractive indices greaterthan about 1.8, and are more effective, generally, as the refractiveindex increases. Titanium dioxides, having refractive indices greaterthan 2, are among the most effective opacifiers in general use, and areoften preferred for that reason.

It has long been known that air, having a refractive index of 1.0, makesa superior interface with opacifiers, and that making paints andcoatings porous by exceeding the "critical pigment volume" or "CPV",loading opacifiers at the surface, and the entrainment of air in thecoating formulation can all enhance the hiding power. This technique, bycreating an interface of air and opacifier, is known to be quiteeffective, but in some contexts results in the compromise of otherproperties of the coating formulation or the resultant coating.

The inclusion of air also enhances opacity through the air-binderinterface, since the binder has refractive index which is materiallydifferent from that of air. This attribute of such systems is lesserthan the air-opacifier interface effect.

The behavior of binders, opacifiers, and the inclusion of air throughone or more of the techniques known to the art are all well known, andare in fact well quantifiable through the application of theLorentz-Lorenz equations and the Fresnel equations. Both diffraction anddispersive effects are accounted for through these techniques. Throughthe application of the techniques known to the art, hiding power of aparticular coating formulation can be predicted quantitatively withconsiderable reliability.

The materials referred to herein as expanded or expandable thermoplasticmicrospheres are most often the materials described in Morehouse, U.S.Pat. No. 3,615,972, and like materials. These materials are per se knownin the art, and do not as such form a part of the present invention.While such materials are disclosed in a substantial number of prior artteachings, the Morehouse patent cited above is the most completedescription of the materials and their formation, and is hence the mostrelevant and material such teaching in relation to the presentinvention.

The microspheres described in the Morehouse Patent have been employed incoatings of a variety of types. Representative of such teachings areWolinski, U.S. Pat. No. 4,006,273, and Wolinski, U.S. Pat. No.4,044,176. These, like all known teachings relating to the inclusion ofmicrospheres in coatings, do not relate directly to opacifiers or hidingpower of the coatings, and are thus materially different from thepresent invention.

The use of opacifiers, as discussed herein, is a vast, well documentedpractice in the art. The use of such materials to render coatings opaqueis not per se a part of the present invention.

According to this invention, traditional Opacifiers are combined withthermoplastic, expandable microspheres under conditions which result inthe solid, opacifiers being adhered to or embedded in the surface of themicrospheres, which are expanded to afford a specific gravity of thecoating formulation and the composite which taken together reduces thetendency for "floating" or "settling". The composite has exceptionalhiding power when formulated into paints. Coatings based on thecomposites have exceptional performance at materially reduced opacifierloadings and cost.

SUMMARY DESCRIPTION OF THE DRAWINGS

The attached single drawing is a microscopic illustration of a typicalmicrosphere-opacifier composite according to this invention.

DETAILED DESCRIPTION

The present invention is based on the employment of a new form ofopacifier. The opacifier of the present invention is a composite ofexpanded thermoplastic microspheres, having adhered to or embedded inthe surface thereof an particulate opacifying component.

The composite is formed by the procedure disclosed in the priorcopending parent applications of Melber, Wolinski and Oswald, citedabove. The disclosure of the foregoing applications are incorporatedherein by reference. The present composite opacifier is a species of theproduct disclosed and claimed in that application, with the followingpoints of distinction:

In the present invention, the solid particulate materials are limited tothose which can serve the opacifying function in paints and relatedcoatings. In the parent, there are many organic particulate materialswhich would not be effective in the present invention, having refractiveindices too low to be effective in the this context.

In the present invention, the proportion of microspheres and opacifyingcomponent, and the extent of expansion of the beads, are carefullybalanced to afford a controlled specific gravity of the composite afterexpansion. This feature reduces the tendency for separation of thecomposite from the paint formulation on standing.

In furtherance of the same objective, the degree of expansion of themicrospheres in the procedure is controlled to afford the targetspecific gravity. This will, in some cases, require some expansion ofthe microspheres, but rather less than the full expansion of which thosematerials are capable.

As used in the present application, microspheres are any of thethermoplastic hollow spheres containing a blowing agent and which arethermally expandable to form light weight hollow structures. Most often,the microspheres of interest are those formed in accordance with theMorehouse Patent cited herein above, which materials are generallyavailable as commercial materials. The commercial versions are made ofpolyvinylidene chloride, and contain alkane blowing agents. Unlessotherwise indicated herein, these are the preferred materials, and arethose referred to, unless otherwise specifically identified as someother material. As an example of such other material, good results havealso been achieved with "ROPAQUE OP-62", manufactured by Rohm and HaasCompany, Independence Mall West, Philadelphia, Penna., under U.S. Pat.No. 4,427,836 --Kowalski, Jan. 24, 1984. As commercially sold, thisproduct has a thin shell of an expandable polymer with a particle sizeof 0.40 microns and contains water as the blowing agent.

In the context of the present invention, it is generally preferred toutilize microspheres of the smallest available sizes, on the order of0.2 to 0.5 microns in diameter.

The opacifier component may be any of the solid particulate materialscommonly employed in the technology of inorganic paint opacifiers. Suchmaterials include rutile and anatase TiO₂, ZnO, CaCO₃ talc, claymaterials, and the like. The particle size requirements of suchmaterials is observed to be of less significance in the presentinvention than in usual circumstances, although it will generally bemost effective to employ particle sizes of diameter near that of thewave lengths of visible light, as is common to the art. Because of theenhanced opacities achieved with the opacifiers of the presentinvention, a desired degree of opacity will often be attained with alesser grade of material, or even a less effective material than isordinarily required in customary formulations.

The characteristics of the microspheres has precluded many approaches totheir drying and pre-expansion. Severe agglomeration and adherence ofthe materials to warm surfaces of equipment have eliminated from seriousconsideration most approaches to such procedures. Wet expansion in steamis of limited use when dry microspheres are needed, and the spray dryingprocedure is so expensive, and the product so prone to excessive, andextremely difficult, dusting problems, that the effective development ofthe potential markets has been limited by such factors.

It has been observed that particulate opacifier components can beemployed, in substantial proportions by weight, which preventagglomeration of the microspheres upon drying and expansion, and thatsuch materials actively and effectively suppress dusting of the expandedproducts as well. This combination of features and observations has ledto the development of effective drying, and optional expansion, ofmicrospheres by mixing such particulate opacifier components into thewet cake, followed by drying, optionally vacuum drying, and recovery ofthe dry, free-flowing product. As an alternative, it is believed that amore reproducible composite can be produced by pre-drying themicrospheres at about 60 degrees C, and them mixing the driedmicrospheres with particulate opacifier. With either approach, themicrospheres remain in the desired unicellular condition, andsubstantially free of undesirable agglomeration. The expansion can be upto the very limits of the microspheres, as established by prior effortsin the art, although the desired specific gravity may often be achievedwith a lesser degree of expansion.

It is important to the present invention that in the context of mostuses of the dry, expanded microspheres, it is the specific gravity orcomposite density considerations which are most often of substantialimportance. Even quite substantial proportions of the particulateopacifier components on a weight basis form a negligible or very minorcomponent on a volumetric basis. For example, employing talc as theparticulate opacifier component, the volume and weight relationships ofthe dry, expanded microspheres with varying amounts of talc show therelationships detailed in TABLE I.

                  TABLE I                                                         ______________________________________                                        EXPANDED MICROSPHERES BLENDED WITH TALC                                       MICROSPHERE CONTENT OF PRODUCT                                                WEIGHT %       VOLUME %                                                       ______________________________________                                        80             99.6                                                           50             98.6                                                           20             94.4                                                           10             88.2                                                            5             78.0                                                            3             67.6                                                           ______________________________________                                         NOTES: Data are based on Microspheres at 0.04 gm/cc and the talc at 2.70      gm/cc.                                                                   

As the relationships in Table I show, even quite large proportions oftalc by weight represent a minor fraction of the volume of the dryexpanded product. It may be advantageous to employ more than one type ofparticulate opacifier component in mixtures and combinations with oneanother.

It has been observed that with appropriate levels of such particulateopacifier components, the tendency of the microspheres to agglomerate,or to stick to heated surfaces of drying equipment is effectivelyeliminated, and the dusting of the final expanded product is materiallyreduced, if not effectively eliminated.

As those of ordinary skill in the art will readily recognize, there area substantial number of parameters which govern the method and theproducts produced in the present invention. Each of the known parametersis hereafter discussed in turn in relation to the present invention.

Microspheres are generally available in the form of a wet cake, which istypically about 35 percent water, about 65 percent unexpandedmicrosphere beads, and minor additional amounts of the materialsemployed in the manufacture of the microsphere beads by the process ofthe Morehouse patent, i.e., "wetting agents."

The most readily available domestic microspheres are those availablefrom Pierce & Stevens Corporation, 4475 Genesee Street, Buffalo, N.Y.,under the trademark "EXPANCEL" which are polyvinylidene chloridemicrospheres with an inclusion of isobutane as the blowing agent.(EXPANCEL is a registered trademark of Casco Nobel AB, a corporation ofStockholm, Sweden.) The available materials are preferred in the presentinvention, primarily for their availability and reasonable cost.

As the Morehouse patent indicates, microspheres can be made from arather wide diversity of thermoplastic polymers. In practice, thecommercially available microspheres are generally limited topolyvinylidene chloride. Microspheres of other materials, such aspolyacrylonitrile, poly-alkyl methacrylates, polystyrene, or vinylchloride, are known, but these materials are not widely and generallyavailable. The present invention is applicable to any thermoplastic ofwhich microspheres is made, but since the polyvinylidene chloridematerials are those most available to the art, the discussion hereinwill be directed predominantly to that material, and to "ROPAQUE OP-62"as mentioned above. As those of ordinary skill in the art will readilyrecognize, the processing parameters will require adjustment toaccommodate differing polymer materials.

A wide variety of blowing agents can be employed in microspheres. Again,the commercially available materials are more limited in range, mostoften being selected from the lower alkanes, particularly propane,butane, pentane, and mixtures thereof, suited to the polyvinylidenechloride polymer. As the Morehouse patent clearly sets forth, theselection of the blowing agent is a function of the particularthermoplastic polymer employed, and in the context of the presentdiscussion, those ordinarily used with the commercially availablemicrospheres are given the greatest attention. Isobutane is most oftenused with polyvinylidene chloride microspheres, while water is theblowing agent in "ROPAQUE OP-62".

In unexpanded form, the microspheres can be made in a variety of sizes,those readily available in commerce being most often on the order of 2to 20 microns, particularly 3 to 10 microns. It has been demonstrated,for example, that microspheres can be made from as small as about 0.1micron, up to as large as about 1 millimeter, in diameter, beforeexpansion. In the present invention, lower particle sizes, i.e. in therange of from about 0.1 to about 10 micrometers, preferably about 0.2 toabout 3 microns, are generally preferred, as an aid in leveling of thecoating formulations into which the opacifier is incorporated.

While variations in shape are possible, the available microspheres arecharacteristically spherical, with the central cavity containing theblowing agent being generally centrally located.

Dry, unexpanded microspheres typically have a displacement density ofjust greater than 1 gm/cc, typically about 1.1 gm/cc.

When such microspheres are fully expanded, they are typically enlargedin diameter by a factor of 5 to 10 times the diameter of the unexpandedbeads, giving rise to a displacement density, when dry, of 0.1 gm/cc orless, often about 0.015 to 0.06 gm/cc.

While the microspheres are produced in an aqueous suspension, it iscommon to break and de-water the suspension, and to supply themicrospheres in the form of a "wet cake." This avoids shipping largerthan necessary quantities of the aqueous system.

The solids content of the wet cake is substantially all unexpandedmicrospheres, but also includes the suspension components, including thewetting agents, so that the remaining water in the wet cake is extremelydifficult to remove.

The present invention is based on the use of conventional contact type,indirect heat exchange mixing driers. A wide diversity of types ofequipment are applicable. In general terms, the requirements are forgood temperature control, good mixing of powder and granular materials,optionally with operation at reduced pressure provided, and the removaland recovery, preferably with condensation of the evaporated water andentrained blowing agent. Cooling of the microspheres, either in themixing drier itself, or in ancillary equipment is also preferred.

There is a great diversity of driers available, at almost any desiredscale of operations which meet the foregoing criteria with a capabilityof either batch or continuous operation in the context of the presentinvention. As a general rule continuous operation is preferred.

Among the commercially available driers with which the present inventionhas been employed are the following:

(1) Luwa Corp: Horizontal Thin Film Contact Driers

(2) Charles Ross & Son: Ross-Bolz Cone Screw Drier

These quite different units have performed quite satisfactorily in thepractice of the present invention, as shown in the examples, infra.

The particulate opacifier component in the present invention is any oneof a wide diversity of materials which meet the requirements of theintended function. It is required that the particulate opacifiercomponent be a free flowing solid at the temperature and pressure of thedrying operation, that it not react chemically with the microspheres, orwith the other constituents of the system, e.g. the wetting agents andrelated components of the wet cake, and that at the temperature of theexpansion, that it function to separate the microspheres undergoingexpansion so that they do not come into contact and bond to one another.It is also required that the particulate contribute opacity to thecoatings to which the composite is incorporated. Substantially all knownopacifiers commonly employed in the coating industry will meet thesecriteria, and thus, can be employed in the present invention with theadaptations required herein.

The particulate opacifier component may be selected from one or morecomponents meeting the following general characteristics:

The opacifier component should be a finely divided particulate solidmaterial, and should be a free-flowing solid under the processingconditions of the present invention. It should have a melting point, forexample, above the temperature of the drying process, generally aboveabout 250 degrees C. Most opacifier materials will have no difficultymeeting this requirement, of course.

The opacifier component must be finely divided enough to be able toeffectively blend with and adhere to the surfaces of the microspheres.The maximum major dimension of the particle size should be no largerthan about the diameter of the expanded microspheres, and preferablyless. The minor dimensions will generally be as small as possible,effectively from about 200 millimicrons or less, up to as much as about2.0 microns. Particle sizes having dimensions near the wave lengths ofvisible light, i.e., about 400 to 800 millimicrons, are particularlypreferred. It is desirable in many situations to employ a blend ofparticle sizes, and it may be desirable in such circumstances to employan increment of the opacifier component having even larger particlesizes as an aid to the drying process as taught in the prior parentapplications.

The particulate opacifier components are desirably materials which areknown opacifiers in coating formulations and thus are commonly used inthe formulations where the microsphere composite materials are to beused. For example, titanium dioxide, talc, calcium carbonate, bariumsulfate, alumina, silica, zinc oxide, mineral clays and the like may beemployed. Other materials of interest may include spherical beads, orhollow beads, of ceramics, quartz, or glass. All these are typical andillustrative of the commonly employed materials in coating compositions,and those of ordinary skill in the art will be familiar with others thatcan also be suitably employed. Blends of such materials can be employedin many cases.

The selection of suitable particulate opacifier components among thewide diversity of materials that meet the general characteristicsrequired of such materials is generally a matter of balancing a numberof functional requirements in the procedure of the invention and in thecontext of the intended uses of the product. Among the criteria thatwill guide those of ordinary skill in the art are the following:

The primary function of the particulate opacifier component during themanufacture of the composite is to prevent the microspheres from cominginto direct contact with one another and with the surfaces of theprocessing equipment while in a tacky, thermoplastic state, and thus toprevent them adhering. The opacifier provides this result by virtue ofadhering to the tacky surfaces of the microspheres as soon as they reacha tacky state, and continuing to adhere throughout the process. Theopacifier component thus becomes adhered to or partially embedded in thesurface of the microspheres, and forms a buffer between thethermoplastic material and any other materials with which it mightotherwise come into contact.

When combinations of different materials are employed as the particulateopacifier component, it is possible to stay within the compoundingrequirements of virtually any designed formulation.

By virtue of the higher density of the particulate opacifier componentthan that of the expanded microspheres, the composite product has agreatly reduced tendency to become entrained in gas streams or in theenvironmental atmosphere. As those of ordinary skill in the art willreadily appreciate, the tendency to dusting is a material safety hazard,both in terms of exposure of workers and in terms of fire and explosivehazards. Since the microspheres may contain an alkane blowing agent insubstantial proportions, large quantities of these materials in theatmosphere presents a substantial problem in some circumstances. Thesedifficulties, and the effort and expense of their resolution areminimized or eliminated altogether in the present invention.

Generally, the greater the density of the particulate opacifiercomponent, the greater the reduction in the dusting problem. Since themajor proportion of the product on a weight basis is the particulateopacifier component, addition of a high density opacifier component tothe system can effectively eliminate any dusting problems.

By virtue of the increased density of the composite, the demands on theprocessing equipment and system in recovering the expanded and driedmicrospheres from fluid streams is greatly facilitated, and productlosses are substantially reduced.

The particulate opacifier component is used in the present invention inan amount sufficient to permit the drying and expansion of themicrospheres without sticking to the equipment employed or formingagglomerations of microspheres. While this amount will vary depending onthe particular equipment employed, and with the particular processingconditions, it will most often be on the range of about 20 to 97 weightpercent of the mixture of opacifier component and microspheres, on a dryweight basis. As a general rule, in most circumstances the amountemployed should be the amount that will reliably and consistentlyachieve the target specific gravity of the composite after any expansionthat may be planned. It is generally preferred that the opacifiercomponent be employed in amounts less than 90, and preferably less than80 weight percent of the blend. This normally results in a dry expandedproduct which is more than 90 volume percent microspheres.

Since the predominant concerns in most uses of microspheres is with thevolumetric proportions, even quite considerable proportions by weight ofthe inorganic particulate opacifier component can be included withoutdetriment in the end uses. When substantial amounts of the particulateopacifier component are introduced as a component of the compositemicrosphere formulation, appropriate allowances for this componentshould be made in the compounding of coating materials. Thus, theproportion of the volatile water or solvent system in the coatingcomposition can often be reduced as well as the proportion of theopacifier agent required for the desired opacity.

In the present invention, contact drying of the microspheres isaccomplished with active mixing, optionally at low pressure, in thepresence of the particulate opacifier component. The term contactheating is employed in the present application to connote heating ordrying involving procedures other than direct heat exchange in a heatedfluid, particularly in a heated gas stream. Contact drying processesemploying indirect heat exchange are generally well known in othercontexts, but in the context of the present invention, must be adaptedto accommodate the particular and unusual conditions of operation, asdescribed infra.

Contact drying, including vacuum drying, is widely practiced for verydiverse and demanding operations which are temperature sensitive.Reducing solutions, suspensions, dispersions, slurries and semisolid wetcake to dry, free-flowing granular solids is commonly achieved in manyindustries with a great diversity of products. There are a substantialnumber of types of equipment in common use, substantially any of whichcan be adapted to use in the present invention. Most such equipmentemploys indirect heat exchange, using steam, heated oil, or the like asa heat transfer medium. Such drying operations commonly employ mixingmeans to distribute the material within the drier, and to preventagglomeration of the material. Reduced pressures range from atmosphericdownward below atmospheric to as low as 1 mm Hg absolute in suchoperations.

Such drying operations are employed in some contexts with thermoplasticmaterials, although not at temperatures at which the thermoplastic meltsor softens, since at a point near the melting point or the glasstransition temperature of thermoplastic polymers, a highly tacky statearises, which would result in severe agglomeration into a relativelymonolithic mass and sticking to the equipment.

It has now been discovered that such equipment can be employed fordrying and expanding thermoplastic microspheres, at temperatures atwhich the thermoplastic material becomes tacky and adherent, by virtueof the action of the particulate opacifier component and the continuousmixing, which combine to prevent sticking to the equipment andagglomeration of the microspheres.

It is also common in such equipment to remove and condense the"distillate" removed from the solid. Since this is done on a continuousbasis, the hazards in the present system as a consequence ofaccumulations of the highly flammable or explosive blowing agent areavoided. The blowing agent, typically isobutane, is continuously removedand condensed in such equipment. This eliminates the need, as has beencommon in the drying of microspheres by spray drying procedures, ofemploying a nonoxidizing atmosphere in the drying chamber. Use of air,or other oxygen containing gases has proved an unacceptable fire andexplosion hazard in such systems, and most are operated by employingnitrogen or some other inert gas as the heat exchange medium. Inert gasdirect heat exchange is quite expensive, and still requires care in thehandling of the substantial gas stream with the blowing agent carriedwith it, and thus solves only a part of the hazard.

The equipment selected for use must, rather evidently, provide foradequate heat transfer to remove substantially all the water from thefeed stock. The significant control parameters for any given equipmentwill be residence time, pressure, and heat input, normally based onoperating temperature for convenience. At the residence time andpressure employed, heat exchange must be accomplished within theconstraints of the temperature limitations of the microspheres, whichcannot be permitted to reach a temperature at which the blowing agentbursts the sphere.

The equipment must also provide the energy for the expansion itself.This is not large, and in most circumstances achieving a beadtemperature (depending on the specific polymer) at which expansionoccurs, as previously defined, there will be little difficulty inattaining the desired degree of expansion. In most circumstances, fullexpansion is desired, i.e., to a microsphere density of less than 0.06gm/cc, preferably about 0.02 gm/cc (without the contribution of theparticulate opacifier component). This refers to the true densities ofthe microspheres and not the bulk or apparent densities.

The important temperature limitations are defined by the thermoplasticpolymer. It is important not to melt the polymer mass, so that thehollow spherical structure is lost through over expansion. On the otherhand, if the temperature is not high enough to soften the polymer and todevelop an adequate pressure of the blowing agent, expansion may notoccur, or may be insufficient. Residence time at the appropriatetemperature is also an important control parameter, since there is adefinite duration for the expansion process. Even when adequatetemperatures are achieved, if the residence time at temperature is tooshort, the expansion may be insufficient. If the time is too long, themicrospheres themselves may be disrupted, leaving broken spheres andpolymer fragments and grit in the product, with attendant losses ofproduction.

As a general parameter, the time and temperature to be achieved isdetermined by the nature of the polymer of which the microspheres aremade, and the degree of expansion required to attain the target specificgravity. The temperatures are generally near, but not materially above,the glass transition temperature of amorphous materials and the meltingtemperature of crystalline polymers. These matters are discussed in moredetail in the Morehouse patent.

It is the function of the particulate opacifier component to prevent theformation of aggregates of the microspheres to the maximum attainabledegree. In most drying equipment this particular requirement isfacilitated by the use of continuous, often relatively high speed, lowshear mixing of the material in the drier. It is worth note thatexcessive shear in the mixing operation may result in disrupting themicrospheres, and must be avoided.

It is generally believed, although applicants have no wish to be boundthereby, that the particulate opacifier components in the presentinvention function to adhere to the surface of the microspheres as theyreach a temperature at which the polymer material becomes tacky. By suchadherence over the surface of the particles, the superficial layer ofthe opacifier component precludes surface bonding between microspheresas they come into contact.

It is one of the unique features of the present invention that themicrosphere beads can be dried without expansion. This has not beenpossible in any effective process in the prior art. Such a result isachieved by drying at temperatures below that at which the microspheressoften, and where the internal pressure of the blowing agent is lessthan that needed to cause expansion. Since the microspheres typicallyexpand at temperatures on the order of about 120 degrees C., drying canproceed effectively at lower temperatures. By use of reduced pressures,the drying can proceed at considerable rates.

The degree of expansion can range from substantially none, to the knownlimits of expansion. This parameter is determined by the temperature,the residence time at temperature, and to a lesser degree, by thepressure in the system. By balancing these parameters against therequirements for evaporating the water, substantially any degree ofexpansion and the targeted specific gravity for the intended use can beattained.

If the particulate opacifier is to be added to wet cake, it is importantto have the opacifier component well dispersed in the continuous phaseduring the drying operation. This requirement ordinarily mandates apre-mixing operation to disperse the particulate opacifier componentinto the wet cake before it is fed to the drier. In some cases, theremay be adequate mixing in the drier to achieve adequate dispersionbefore the point at which the drying proceeds to the extent thatrequires uniform dispersion, but in most circumstances, those ofordinary skill in the art will recognize, a pre-mixing step will insurebetter results. It will generally not be necessary to add wetting agentsor surfactants into the mixture in order to attain adequate dispersionbecause of the wetting agents already present in the wet cake.

The microsphere beads expand at a temperature which is a function of thespecific polymer and blowing agent employed. Typically, expansion occursat about 120 degrees Centigrade. At reduced pressure, expansion mayoccur at slightly lower temperatures.

Expansion requires that the blowing agent develop a substantial internalpressure (as compared with the external pressure), and that the polymerbecome softened enough to flow under the effect of the internalpressure. This generally means that the polymer must be heated to apoint near its melting or glass transition temperature, or very slightlyabove. If the polymer temperature is too high, the microspheres willoverexpand, burst, and collapse. The range of actual temperaturesnecessary will depend upon the specific microspheres utilized, i.e. thepolymer therein. At temperatures near the upper limit, the residencetime at temperature should be brief.

It will often be desirable to conduct the drying operation at reducedpressure to accelerate the rate of the water removal. Thus, in thepresent invention, pressures from ambient to as low as 1 mm Hg absolutehave been employed with success. As those of ordinary skill in the artwill readily recognize, the balancing of time, temperature, and pressurecan be readily adapted to the substantially complete removal of thewater and the appropriate expansion of the microsphere beads.Particularly when little or no expansion is wanted, low pressure dryinggreatly facilitates low temperature operations at which the expansion ofthe microspheres does not occur.

As the temperature is raised to the point at which the microspheresbegin to soften and expand, and their surface area becomes tacky, theparticulate opacifier component will adhere to the surface. Good mixingoperates to maximize the extent of contact between the particulateopacifier component and the microspheres at this stage in the process.The extent of the mixing is not narrowly critical, so long as arelatively homogeneous dispersion of the opacifier component and themicrospheres is maintained, and so long as the mixing does not disruptthe structure of the microspheres.

It is generally preferred to actively cool the dried and expandedmicrospheres before they are collected and packaged or otherwisehandled. When reduced pressure is employed in the drier, it is preferredthat the microspheres be stabilized by cooling before the pressure isincreased. This minimizes the degree to which the pressure change canoperate on the polymer and possibly disrupt the system while the polymeris in the plastic state.

The resulting dry microspheres can be conveniently recovered from thedrier, collected and handled by entirely convention procedures andequipment usually employed in such drying operations for dealing withpowdered or granular materials.

The result of the process is the production of a unique form of thecomposite opacifier-microspheres. The composite will comprise themicrospheres which have an adherent surface deposit of the particulateopacifier component, ordinarily adhered to or partially embedded in thesurface of the polymer material. When an excess of the particulateopacifier component is used, there may be an additional amount of freematerial entrained in, but not bound to the surface of, themicrospheres. The particulate material may form a discontinuous layer onthe surface, or in other circumstances may completely coat the surfacein a continuous layer. By varying the proportions of the opacifiercomponent and the microspheres, either condition may be attained.Depending on the intended environment of use, either condition may bepreferred. For example, when the microspheres are to be incorporatedinto a coating polymer matrix which does not readily wet and bond to thepolyvinylidene chloride, the adhered or embedded particles of theparticulate opacifier component can function effectively as a "primer"or "key" coating on the beads, resulting in improved bond strength insuch circumstances. In other cases, where the polymer binder formsstrong bonds directly to the polyvinylidene chloride, a discontinuouscoating of the opacifier component may result in better bonding.

The composite opacifier-microspheres of the present invention areessentially a dry, free-flowing powder, but can contain up to about 5 %moisture and retain its free flowing characteristic. Because there willstill be a residuum of the "wetting agents" remaining from the limitedcoalescence process by which the microspheres were made, the productwill be slightly hygroscopic, and unless protected from ambientmoisture, will gradually take up additional water. The materialsinvolved are not so strongly hygroscopic, however, that this is a majorproblem. In most circumstances, unprotected microspheres will tend tostabilize at a water content of about 1.5 weight percent. Themicrospheres will remain a free flowing powder even under suchconditions. When formulating aqueous based coatings, the moisturecontent of the opacifier can generally be ignored.

The microsphere product of the present invention can be unexpanded, orcan be expanded to very near the limit of expandability, i.e., to adensity of between 0.010 and 0.015 gm/cc. Intermediate values are alsopossible. When the particulate opacifier is taken into account, thecomposite density will, of course, be higher, and should have a densitywithin the broad range of 0.1 to 2.8 gm/cc, and preferably within therange 0.15 to 1.5 gm/cc to make a non-floating, non-settling product.Utilizing "ROPAQUE OP-62", excellent composite has been produced havinga density of 2.8 gm/cc. Thus the composite density of the product willbe determined by the density of the particular opacifier componentemployed, the amount of the opacifier component included, and the degreeof expansion. Those of ordinary skill in the art will be able to readilydetermine the composite density of the product from the informationprovided in Table I, hereinabove.

The opacifier component of the composite is adhered to or embedded inthe surface of the microsphere, and is believed not to be in directcontact with the gases on the interior of the microsphere structure.Thus, so far as is presently known, there is no opacifier-air interfacepresent. In that context, it has been surprising to observe that theperformance of the composite is fully equivalent to that which would beanticipated if there were such an interface. Observed hiding power ofthe formulations with the composite of the present invention aretypically about 225 to 250 percent of the values predicted by theLorentz-Lorenz equations for titanium dioxide-polyvinylidene chlorideinterfaces, and are fully equivalent to those anticipated for titaniumdioxide-air interfaces. The degree of enhanced hiding power is evengreater for other, less efficient opacifiers. In the case oftalc-microsphere composite opacifier, for example, the hiding power isoften in excess of 100 times that for the talc alone.

These observations are not fully understood, and have been incompletelyinvestigated at the present, but it is believed possible that theexpansion of the microspheres preferentially reduces the thickness ofthe polyvinylidene chloride film at the points at which the embedmentsof the solid particles are located, so that the film is reduced todimensions of thickness substantially less than the wave length ofvisible light, where refractive behavior is substantially altered, andin the present context ceases to be a material factor in the refractioncharacteristics of the composite. The thickness of the microsphere wallhas been observed to be on the order of about 30 to 50, typically about40 millimicrons, while the wave lengths of visible light are, of course,from 400 to 800 millimicrons.

As a consequence, the behavior of the opacifiers in the presentinvention is fully consistent with the attainment of high levels ofporosity and air entrainment in coatings, fully equivalent to thebehavior of such systems with opacifier loadings substantially in excessof the "CPV" even when materially less opacifier than that required toattain the CPV is employed. On the other hand, the deleterious effectson the performance of coatings as a consequence of the entrainment ofair and the introduction of porosity is substantially completelyavoided.

The composites are compatible with a very wide and broad diversity ofpaint binders, vehicles, and ancillary components. The sole limitationsare that the formulation must not dissolve the material of themicrospheres (which may in some cases be assured by treating themicrospheres to assure such a result), and the conditions of the makingup of the formulation must not disrupt the physical structure of themicrospheres. Hence, if any components require grinding or milling,those steps should be conducted before the inclusion of the microspherebased composite. The opacifying constituents are quite insensitive tosuch matters and do not pose any limitations on the make up of theformulations into which the composite is added.

The composite opacifiers of the present invention are readilyincorporated into paints, inks, and other coating formulations withlittle effort or difficulty. When the vehicle of the formulation wetsthe polyvinylidene chloride or the opacifying constituent, the compositewill be quite readily dispersed in the formulation with nothing morethan simple mixing. In an aqueous media, of course, the wetting agentsemployed in the manufacture of the microspheres will generally remainpresent, so that dispersion in aqueous systems is generally quitesimple. In other vehicles, it will ordinarily suffice to wet thecomposite with a surfactant or wetting agent appropriate to andcompatible with the vehicle.

In some cases, it may be preferred to formulate the composite into abase with the coating binder, vehicle, and other suitable ingredients,designed to be mixed with other components which are formulatedseparately, such as color concentrates, and the like.

The specific gravity of the composite opacifier is regulated, bydetermining the relative proportions of microspheres and components andthe degree of expansion of the microspheres to provide a specificgravity which approximates that of the coating vehicle. By this featureof the invention, the tendency of the opacifier components to separatefrom the coating formulation is very slight or non-existent. This is aparticular advantage, as those of ordinary skill in the art will readilyrecognize.

As a consequence of the high volume of the composite in relation to theamount of the opacifying power, and the hiding power of the coatingformulation into which it is incorporated, it has been found that theweight per unit volume for a given hiding value will be greatly reduced,and in substantially all cases will be less than the CPV or "criticalpigment volume". While there is no fundamental reason the CPV of thecoating system may not be exceeded, there will rarely be any benefit indoing so when utilizing the composite of this invention. Furthermore,some of the advantage of the present invention, predicated on theavoidance of the introduction of porosity and the physical effects onthe coating film will be lost by exceeding the CPV, so that such loadinglevels will not often be desirable for economic reasons and for thedegradation of the coating that comes with such procedures. Since weightis a primary characteristic of paints and coating formulations, therecan be material savings as a consequence of these effects, in reducingshipping and handling costs, and in affording greater ease of handlingof such materials.

In light of the enhanced performance of the opacifiers of the presentinvention when contrasted to the opacifying component as used alone inthe prior art, it is often possible to reduce the amount of suchcomponent by a material proportion, often as little as half or less ofthe amount formerly required being effective to attain the same opacityand hiding power. It is also frequently possible to employ a lessexpensive grade of opacifier material, or even a different material oflower opacifying capacity, as a consequence of the enhancedeffectiveness of the composite form. Since the opacifiers are, as ageneral rule, the single greatest cost component of paints and othercoating formulations, the savings made possible by the present inventioncan be quite substantial. The inclusion of the microsphere component isa quite minor and modest cost factor in relation to the savings madepossible by the reduction of the titanium dioxide, say, by one half orthereabouts, or the substitution, in whole or in material part, ofinexpensive talc for titanium dioxide.

It is of course necessary to take appropriate steps to adapt thecomposite to the specific formulation into which it is to be formulated.As those of ordinary skill in the art will readily recognize, thepolymer of the microsphere component must not be soluble in theformulation or any component therein. For most common paint vehicles,this is a readily attained objective.

TESTS AND EXAMPLES Test 1

As already noted, the use of the composite opacifier-microspheres inpaints and other coatings has given greater than predicted opacity. Thissynergistic effect may be due to the light scattering power of themicrospheres themselves, the dilution effect of titanium dioxide (Aboveten volume percent titanium dioxide the efficiency of hiding isdrastically reduced), or the unexpected synergism of titanium dioxideattached to the microspheres, or a combination of all or any of theseeffects.

In order to obtain a quantitative measure of the synergistic effect, thefilm scattering coefficient of several samples were first determined,and then these values contrasted to those predicted so that the percentenhancement in opacity could be calculated. The film scatteringcoefficient of seven test samples were determined using the proceduredescribed in the Official Digest, Sept. 1963, pp. 871-911, P. B. Mittonand A. E. Jacobson, giving a Kubelka-Munk scattering coefficient inmil⁻¹. Table II below lists the values for the scattering coefficientsfor the seven samples tested.

                  TABLE II                                                        ______________________________________                                        FILM SCATTERING COEFFICIENT (Mil.sup.-1)                                      OF SEVEN TEST SAMPLES                                                         Scattering                                                                    Coefficient Titanium Dioxide                                                                           Microspheres                                         (Mil.sup.-1)                                                                              Volume Fraction                                                                            Volume Fraction                                      ______________________________________                                        4.52        0.120        0                                                    8.34        0.215        0                                                    8.78        0.291        0                                                    1.76        0            0.350                                                1.60        0            0.410                                                7.34        0.075        0.405                                                9.01        0.070        0.520                                                ______________________________________                                         From Table II can be seen the decreasing efficiency of titanium dioxide as     the volume fraction increases. A similar effect can be seen for volume     decreases in microspheres.

When two scattering particles are present, the contribution of eachparticle should in theory depend on the volume fraction of eachmultiplied by the scattering coefficient. The equation for this is S=S₁V₁ +S₂ V₂, where S is the scattering coefficient and V is the volumefraction present. Using this equation one can calculate the individualscattering contributions of each constituent and the theoretical total.By contrasting this total to the above values, any synergistic effectcan be determined. The film scattering coefficients for titanium dioxideand for microspheres was determined over a volume percent of 0.05 to0.60 to be 38 and 6 mil⁻¹ respectively. For the sample 6 composite inTable II, S×(0.075)(38) +(0.405)(6)=2.85 +2.43 =5.28. On the other hand,the value measured as shown in Table II was 7.35. Therefore, thesynergistic effect was 7.35-5.28=2.07. Accordingly, the percentenhancement in opacity was (2.07/2.85)100%=73%, while the totalenhancement ratio over that of titanium dioxide was(7.35/2.85)100%=258%.

Similarly for sample 7 composite in Table II, the calculations show:S=(0.07)(38)+(0.52)(6)=2.66 +3.12 =5.78 mil⁻¹ However, the measuredvalue was 9.01. Therefore, the enhancement in coefficient was9.01-5.78=3.23. Accordingly, the enhancement in opacity was(3.23/2.66)100%=121%, while the total enhancement ratio over that oftitanium dioxide was (9.01/2.66)100% =399%.

Test 2

The lower value for the scattering coefficients for microspheres and theinventive composites with microspheres in contrast to titanium dioxide,requires that a thicker film be used to obtain 98% hiding powertypically required for paint films and other coatings. A contrast ratioof 0.98 is required where the contrast ratio is defined as the ratio ofthe reflectance over a black/white Leneta chart. Judd and Wyszecki intheir book Color in Business, Science and Industry, Wiley, N.Y, N.Y.,1963, defined equations which allow film thicknesses, X, to becalculated if the film scattering coefficients, S, are known, where filmreflectivity R=94% and substrate reflectivity R_(s) =80%.

Rearranging the book's equation for ease of calculating gives thefollowing equation:

X=(R -R_(s))/S(1-R_(s))(1-R₀)

The Kubelka-Munk equation was derived with terms that quantize theabsorption of incident energy. The average of visible light, 0.55microns, was used for this equation. Titanium dioxide and microspheresdo not absorb energy at this wave length. Terms relating to theabsorption of energy in the original Kubelka-Munk equation wereeliminated to permit use of the simplified equation shown above. TableIII below provides film thicknesses required for 94% hiding when thescattering coefficient is known. These data are for film reflectivity of94%, substrate reflectivity of 80%.

                  TABLE III                                                       ______________________________________                                        FILM THICKNESS FOR                                                            98% HIDING POWER                                                              Scattering                                                                    Coefficient                                                                             Film Thickness          Volume                                      (Mil.sup.-1)                                                                            (mils)        Material  Fraction                                    ______________________________________                                        38        0.61          TiO.sub.2 0.50                                        6         3.89          MS        0.50                                        7.34      1.59          Sample 6  Table II                                    9.01      1.29          Sample 7  Table II                                    ______________________________________                                         Different film thickness would be obtained with different reflectivities.

Test 3

In another study, a 50PVC paint was made using the compositeopacifier-microspheres (50PVC O/MS) and compared to a 50PVC prior artpaint using only titanium dioxide (50PVC TiO₂). The compositeopacifier-microspheres, identified as "M6018", was a composite oftitanium dioxide/microspheres at a ratio of 95.46/4.54. Table IV belowshows how they compared.

                  TABLE IV                                                        ______________________________________                                                      50 PVC O/MS                                                                             50 PVC TiO.sub.2                                      ______________________________________                                        Grind                                                                         Propylene Glycol                                                                              8.1         5.8                                               Tamol 850       0.9         1.9                                               Colloid 643     0.28        0.2                                               TiO.sub.2                   43.6                                              M6018 (O/MS)    23.7                                                          H.sub.2 O       19.2        6.93                                              SB8208          0.017                                                         M73             0.003                                                         Texanol         1.8                                                           Letdown                                                                       Rhoplex E1953   38.7        29.3                                              H.sub.2 O                   4.1                                               Texanol                     0.8                                               Nuosept 95      0.1         0.08                                              Triton GR-7M    0.1         0.08                                              Thickener                                                                     H.sub.2 O       6.9         7.1                                               Natrosol 250 MHBR                                                                             0.2         0.2                                                               100.00      100.00                                            Hiding Power 1 mil dry                                                                        85          97                                                Volume Solids % 35          35                                                Density Opacifier gm/c                                                                        1           4.2                                               Thickness for 98%                                                             Hiding Power    1.90        1.0                                               ______________________________________                                    

Resistance to loss of hiding and opacity was measured with water. Thedry film, about 2 mils dry, was applied to a Leneta chart. After threedays the contrast ratio was 98%. The samples were then covered withwater and a watch glass was placed over the water to preventevaporation. After 24 hours, the watch glass was removed and the hidingwas measured. A reading ratio of 98% indicated no loss of hiding oropacity as is experienced with coatings that have "dry " hiding, wherethe critical pigment volume has been exceeded and where air is present.Replacement of air by water causes loss of hiding in other paint films.

EXAMPLE 1

An alkyl paint was made using the M6018 composite of this invention asfollows:

    ______________________________________                                        M6018            24.0%                                                        Soya Alkyd Resin 28.8%                                                        Drier            00.4%                                                        Mineral Spirits  46.8%                                                                         100.0%                                                       ______________________________________                                    

This was a solvent based paint, 40 PVC. 98% hiding required a coating ofabout 2.6 mils dry.

EXAMPLE 2

A coating mixture was prepared as follows:

    ______________________________________                                               M6018     20.0%                                                               Polyamid Resin                                                                          20.0%                                                               Urea Resin                                                                              00.6%                                                               Mineral Spirits                                                                         46.2%                                                               Glycol Ether                                                                            13.2%                                                                         100.0%                                                       ______________________________________                                    

The activator was prepared as follows:

    ______________________________________                                        Epoxy Resin        61.2%                                                      Propylene Glycol Ether                                                                           38.8%                                                                         100.0%                                                     ______________________________________                                    

Equal parts of the coating and the activator were mixed thoroughly andallowed to pre-react for one hour. A coating was applied by brush to adry thickness of about 4 mils. Hiding was 98% after a cure for threedays.

What is claimed is:
 1. The method of making a composite opacifier forcoatings, comprising:A. mixing an particulate inorganic coatingopacifier component with expandable thermoplastic resin microspheres,said opacifier component comprising from 20 to 97 weight percent of themixture, with a material proportion of said opacifier component having aparticle size of from about 200 to about 2000 millimicrons; B. heatingthe mixture of said opacifier component and said microspheres underconditions of time, temperature and pressure to cause said opacifiercomponent to be embedded in and adhered to the surface of saidmicrospheres; C. thermally expanding said composite to attain acomposite specific gravity of from about 0.1 to about 2.8 gm/cc; D.recovering said opacifier for incorporation into coating formulations.2. The method of claim 1 wherein said opacifier component is added toand mixed with expandable thermoplastic resin microspheres, underconditions of time, temperature and pressure to remove substantially allmoisture from said mixture and causing said opacifier component toadhere to the surface of said microspheres.
 3. The method of claim 1wherein said composite is thermally expanded to attain a compositespecific gravity of from about 0.15 to 1.5 gm/cc.
 4. The method of claim1 wherein a material proportion of said particulate coating opacifierhas a blend of particle sizes within said range.
 5. The method of claim1 wherein a material proportion of said particulate coating opacifierhas a particle size of from about 400 to 800 millimicrons.
 6. The methodof claim 5 wherein a material proportion of said particulate coatingopacifier has a blend of particle sizes within said range.